Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

Abstract

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe­(TPP)­X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are <i>D</i> = 4.49(9) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­F], and <i>D</i> = 8.8(2) cm^–1, <i>E</i> = 0.1(2) cm^–1 and <i>D</i> = 13.4(6) cm^–1, <i>E</i> = 0.3(6) cm^–1 for monoclinic polycrystalline [Fe­(TPP)­Br] and [Fe­(TPP)­I], respectively. Along with our recent report of the ZFS value of <i>D</i> = 6.33(8) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe­(TPP)­X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting <i>D</i> of the ⁶A₁ ground state. <i>D</i> was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe­(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of <i>D</i> from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the <i>S</i> = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆^3– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of <i>D</i> and decrease of the π- and σ-antibonding energies <i>e</i>_λ^X (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. <i>D</i> parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental <i>D</i> and spectroscopic transitions for high-spin Fe­(III) complexes are proposed